Did some research and I think I read some where that it's recommended to have a separate phone for hobbying. Is that really necessary or just having a separate SIM card enough? Like put the hobby specific SIM card in when I want to hobby and take it out when I'm done

I'm a complete newbie myself but,  I have a separate number myself just to be safe.

If you have a smartphone there are plenty of apps you can download which allows you txt and call from another number.

...on this topic. Use hobby phone and variations as search words. Search on the Gen. Dis. board too. In the number of days field put 300 or 600 so you'll get plenty of discussions on the subject.

Separate Sim card works as an emergency measure. But, a lot get stored on the phone (phone logs, texts, etc.). You have to clear the phone after each use with the hobby sim. One more thing to forget.

Significant other finding your call/text records or billings?  Get a separate pay-as-you-go phone and learn to hide it.  Or get a separate SIM to swap in current phone and learn how to purge records between swaps.  Or tell the SO and let the chips fall where they may - but odds are you already know how that'd pan out!

Providers knowing your phone number?  Either of the above or use a burner app to generate disposable phone numbers on your current phone.

Law enforcement accessing your call/text records?  Any of the above plus encrypting device with a strong passphrase will slow them down, but won't stop them depending on the agency.  Depending on your phone service the network already has that on file and likely turned it over to LEO anyway.  Take care with what you text to minimize that risk, but as of now it's not illegal in the USA to call/text someone regardless of their endeavors.

It's really difficult to regain privacy once you've openly used a phone number; but much easier to decide later on that you don't need a separate phone/SIM/burner and then stop using it.

I can tell you from experience. My hobby contacts ended up in my civvie job Outlook contacts on my work computer. I have accidentally emailed clients from my firstnamelastname@gmail instead of my provider email. I almost emailed my dad from my provider email. All on my iPhone.

So many stores have the super cheap pre-paid phones. It would be worth it, especially if you're married.

that if you have a spouse or SO, you should have a burn phone and email address for hobbying only.  I'm single with no SO, so I don't use a hobby phone.

Swim

Still relatively new myself, but the OP should decide what they're trying to do.  I now think there is a tradeoff between the best thing for hiding from LE and the best thing for hiding from SOs or general practicality.

My understanding of LE best practice is a pre-paid burner phone paid for and reloaded in cash.  Hobby email setup on one of the sites that doesn't IP address log.  Ideally only access hobby related things through TOR running on a virtual machine running a linux distro or the amnesiac operating system.  Never use a credit card for anything in this identity.

Some of the recommendations in that paragraph are difficult to implement as a practical matter.  For example, I've heard changing phone numbers regularly makes providers uncomfortable as might hushmail addresses (anyone confirm or deny?).  There's also the issue of making sure your burner prepaid phone is somewhere your SO will never find it (yes, hide it at work but you may slip up).

Provided you're willing to accept marginally greater LE risk, alternative sims/burners apps might make sense.  A lot of providers also use gmail and they aren't running .onion sites, so there's probably a functional limit to how far you can take the digital track hiding stuff.  So, basically try to figure out what you're comfortable with.  I have a few heuristics for LE risk minimization, but lately I've decided practicality matters too in ways that weren't as apparent very early on (ex. booting a disposable VM just to check my email is a pain).

Eeny, meeny, IMEI, mone.
Catch a hobbyist by the phone.
If he hollers, let him phone  
His lawyer on the following morn.

I'm no tech expert, but every phone has an IMEI number, a kind of permanent unique hardware address.  That is encoded in the signal when you use the phone.  One phone, multiple SIM cards, ONE IMEI connects them all.

I recommend SEPARATE hardware, separate phones, separate SIMs, ... keep it all separate

I went with a spare sim card, but that is only because i have an old iPhone and galaxy SII collecting dust in a drawer.  The SO will not give a second thought to seeing my old phones laying around.  I do remember to take the sim card out however and hide it in another location.  Its current home is my GS2 that is missing an OS, so even if my wife turned the phone on, it wouldn't get passed the Samsung screen.    A separate hobby phone just seemed like too much of a risk.   I'm pretty techy and my SO is...not.  So she would think nothing of yet another sim card or random electronic component hanging out in my desk drawer. That being said, the sudden appearance of a brand new cell phone, especially one labeled TRACFONE would cause a major shitstorm.